1. Field of the Invention
The invention relates to a transparent, shrinkable film, consisting of one or several layers, in which the one layer or, in the case of several layers, at least one of the layers consists of polyamide.
2. Description of the Prior Art
Polyamide films are known as single layer as well as multilayer films and are in use on a large scale in the packaging industry, especially for the packaging of foods. Polyamide films are used especially because of their great strength and their low permeability to oxygen. In conjunction with polyolefin films, that is, as multilayer films, they have, besides the high strength, good heat sealing properties which, in conjunction with the high melting point of the polyamide, lead to an easy processibility and secure sealing. In this connection, the polyolefin layer is taken as the inner layer of the packaging material, that is, the easily sealable layers lie against each other, while the polyamide layer, as outer layer with the higher melting point, prevents adhesion to the hot sealing block. Moreover, polyolefin layers have a significantly lower moisture permeability than polyamide, so that the two plastics complement each other outstandingly and the multilayer film of polyamide and polyolefin is almost the ideal packaging material film. The disadvantges of previous polyamide films as shrinkable films, irrespective of whether they are used as multilayer films or as single film, are due on the one hand to their inadequate shrinkage behavior or their slight stretchability at justifiable temperatures and, on the other hand, to their frequently deficient transparency.
Various proposals have therefore already been made for using polyamide films for shrinkable packaging material or to adapt it to this purpose. For example, German Auslegeschrift No. 1,769,510 describes a multilayer film of polyamide and polyethylene made by the coextrusion process, in which the film may be stretched monoaxially or biaxially. German Auslegeschrift No. 2,352,553 uses a similarly constructed multilayer film of linear polyamide or linear copolyamides and polyethylene as sausage skin, the sausage skin being stretched in the longitudinal direction in order to improve the shrinkage. According to German Offenlegungsschrift No. 2,850,181, a tubular film of a mixture of an aliphatic, that is, a linear polyamide and an ionomeric resin and/or modified ethylene/vinyl acetate copolymers is proposed for the same purpose. With the exception of special purposes, none of these proposals has been successful in the market, because the shrinkability of these films is too slight, the expenditure for the work of stretching is large and moreover because they have an inadequate transparency.
The shrinkable film, described in German Pat. No. 1,461,842, is built up from aromatic polyamide components such as terephthalic acid and branched aliphatic diamines, that is, it consists of a partially aromatic polyamide, which is stretched at temperatures of 150.degree. to 160.degree. C. This film has a better shrinkage behavior, but has the disadvantage that the stretching temperatures, which must be used, are very high and, above all, that the shrinkage temperatures of 170.degree. to 190.degree. C., which must be used, are too high for most of the goods, which are to be shrink-wrapped, and harm the material packaged.
Apart from shrinkability, further essential requirements, which such a packaging film must fulfil, are a good transparency, without at the same time being brittle. As far as possible a packaging film should be crystal-clear and glossy in order to set off the materials packaged therein as advantageously as possible. At the same time, it must not be brittle, as is the case with the film of German Pat. No. 1,461,842, so that there occurs no damage to the film or the goods themselves due to the action of mechanical stresses, for example, during transport.
In order to improve the transparency, it has already been proposed that an amorphous structure be built up by the polycondensation of different monomers. For example, a composite film of polyamide and polyethylene was proposed in German Offenlegungsschrift No. 2,309,420, in which the polyamide base consists of copolymers of .epsilon.-caprolactam, 3-aminomethyl-3,5,5-trimethylcyclohexylamine and isophthalic acid, the proportion of the latter being less than 15 weight percent.
German Offenlegungsschrift No. 2,538,892, which also relates to a multilayer film of polyethylene and polyamide, has in mind that, as polyamide, a copolyamide of .epsilon.-caprolactam and AH salt--a polycondensate of hexamethylenediamine and adipic acid--is coextruded together with polyethylene. The objective of both proposals is to improve the transparency. These multilayer films with a polyamide layer of a copolymer of the above-given monomeric components however have inadequate shrinkage capabilities.
The state of the art accordingly does give advice here and there how the one or the other property, for example the shrinkability or the transparency, can be improved. There is, however, no indication in any of the publications in respect to how a film may be produced, which can be shrunk at temperatures which are economically feasible and do not damage the goods, and nevertheless retains all the remaining desirable properties, such as transparency, impermeability to gas, etc.